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Comments
on Exaggerated Channel Cross-sections
I teach three levels of intensity of
stream geomorphology to students in local, state, and federal agencies,
and like proud pups they come up with exaggerated cross-sections
and longitudinal profiles to present to me. True to these individual’s
skills, their engineering and surveying notes are excellent; yet,
they miss the intimate form and process of bankfull channel dimensions
and their width to depth/entrenchment ratio characteristics which
are pivotal in planning.
Although most profiles of reasonable
distances may need some exaggeration, cross-sections rarely do.
I personally, could not have better-stressed the point you made
about understanding the true fluvial form of the bankfull discharge
and its relative connection to the floodplain. It's a shame to
go to the work of profiles and particularly cross-sections and then
lose the visual effect of a land form on paper.
I too have found the 1:1 scale superior
for most planning and eventually design reasons. I also find that
trackhoe operators relate better to non-exaggerated scales of cross-sections
and slightly exaggerated profiles. Yes, we use up more paper and
wind up folding them to fit in design and planning reports, but
it is a more honest representation of form.
The one use I have found legitimate
for exaggerated ordinate axis on cross-section has been in the comparisons
of permanent monitoring and assessment plots on streams of interest,
over shorter periods of time. One can see more subtle changes,
particularly on larger rivers regarding depth changes. However,
I always like to include a 1:1 just above or to the side of the
exaggerated plot to keep the dimensions well defined.
W. Barry Southerland
Stream Geomorphologist, USDA-Natural Resources Conservation Service,
Washington State Office, Spokane (barry.southerland@wa.usda.gov).
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